The present invention relates, in general, to a frequency discrimination circuit, and more particularly, to a frequency discrimination circuit used for speed detection in a speed control device.
In an electric apparatus which includes a rotating body required to have a constant speed of revolution such as rotating heads and capstans of video tape recorders (VTRs) a so-called servo control system is generally employed in which the speed of revolution of the rotating body is detected and a motor for driving the rotating body is controlled by the detected speed signal so that a predetermined speed of revolution is maintained.
An example of the above-mentioned servo control system is shown in FIG. 1 of the accompanying drawings. Referring to FIG. 1, the speed of revolution of a rotating body 2, for example, the rotating head of VTR driven by a drive motor 1 is detected by a detector 3, which delivers a pulse signal A having a frequency proportional to the revolution speed of the rotating body 2. The pulse signal A is applied to a frequency discrimination circuit 4, which discriminates frequencies and delivers an error voltage E corresponding to a deviation of the frequency of the pulse signal A from a predetermined frequency, that is, a deviation of the actual speed of revolution from a predetermined one. The error voltage E is applied to a motor driving amplifier 5, the output of which is used to control the speed of revolution of the drive motor 1. Thus, the revolution speed of the rotating body 2 is maintained at a predetermined value.
As is apparent from the foregoing, in the servo control system, the control characteristic of the whole system is greatly affected by the performance of the frequency discrimination circuit 4. Accordingly, there have been proposed various types of frequency discrimination circuits, a typical one of which includes a plurality of monostable multivibrators (hereinafter referred to as "MMV"). An example of such a frequency discrimination circuit is shown in FIG. 2. In FIG. 2, reference numeral 101 designates an input terminal applied with the pulse signal A which is shown in FIG. 1 and indicates a speed of revolution, 102 an output terminal for feeding the error voltage E shown in FIG. 1 to the motor driving amplifier 5, 103 and 104 monostable multivibrators having time constants .tau..sub.1 and .tau..sub.2, respectively, 105 a trapezoidal wave generator, 106 a pulse shaper, and 107 a sample-and-hold circuit. The sum of the time constants .tau..sub.1 and .tau..sub.2 is used to determine a reference period.
Next, the operation of the frequency discrimination circuit shown in FIG. 2 will be explained using a waveform chart shown in FIG. 3. The pulse signal A from the input terminal 101 is applied to MMV 103 and the pulse shaper 106. MMV 103 is triggered at the leading edge of each input pulse A to generate a square wave signal B having a pulse width .tau..sub.1. MMV 104 is triggered at trailing edges of the square wave signal B from MMV 103 to produce a square wave signal C having a pulse width .tau..sub.2. At trailing edges of the square wave signal C from MMV 104 the trapezoidal wave generator 105 is triggered to deliver a trapezoidal wave signal D which has a constant inclination part of a period .tau..sub.0, that is, a ramp characteristic. The signal D is applied as the sampled input to the sample-and-hold circuit 107.
The angle of inclination of the constant inclination part determines the frequency discrimination sensitivity. In more detail, when the angle of inclination is too large, the error voltage E takes only two levels, that is, high and low levels. Accordingly, the above-mentioned control system performs an undesired on-off control.
The pulse singal A applied to the pulse shaper 106 is shaped into a pulse signal P which is synchronized with leading edges of the pulse signal A and has a relatively narrow pulse width. The pulse signal P thus shaped is applied as the sampling input to the sample-and-hold circuit 107. The circuit 107 delivers the error voltage E, which holds during a sampling period a value sampled from the trapezoidal wave signal D by the pulse signal P, for the reasons described later. When a reference period corresponding to a predetermined target speed of revolution is given by T.sub.0 =.tau..sub.1 +.tau..sub.2 +.tau..sub.0 /2, where .tau..sub.1, .tau..sub.2 and .tau..sub.0 indicate the time constant of MMV 103, that of MMV 104 and the period of the constant inclination part of the trapezoidal wave signal D, respectively, and when the frequency of the pulse signal A is expressed by T, the error voltage E takes a voltage value E.sub.0, which is equal to about one-half a maximum value of the trapezoidal wave signal D and indicates that the deviation is equal to zero, if T=T.sub.0. Further, when the revolution speed of the rotating body 2 shown in FIG. 1 is reduced to less than the predetermined value, that is, T&gt;T.sub.0, the error voltage E becomes greater than the voltage E.sub.0. While, when the revolution speed of the rotating body 2 is increased to more than the predetermined value, that is, T&lt; T.sub.0, the error voltage E becomes smaller than the voltage E.sub.0. These changes in error voltage are shown as stepwide changes of the error voltage E relative to the voltage E.sub.0 in FIG. 3.
The error voltage E is applied to the motor 1 through the motor driving amplifier 5 (as shown in FIG. 1). When the revolution speed of the rotating body 2 is reduced, the error voltage E becomes greater than the voltage E.sub.0. Accordingly, the voltage applied to the motor 1 is increased so that the revolution speed of the rotating body 2 is enhanced. On the other hand, when the revolution speed of the rotating body 2 is increased, the error voltage E becomes smaller than the voltage E.sub.0. Thus, the voltage applied to the motor 1 is decreased so that the revolution speed of the rotating body 2 is lowered. In other words, the revolution speed of the rotating body 2 is controlled so as to be maintained at a predetermined value, at which the period T of the pulse signal A is equal to the reference period T.sub.0 and the error voltage E is equal to the voltage E.sub.0.
Since the above-mentioned conventional frequency discrimination circuit is relatively simple in construction and has satisfactory sensitivity and linearity with respect to frequency discrimination from a practical point of view, it has been widely employed.
In the above frequency discrimination circuit, however, a predetermined speed of revolution of the rotating body is set by those time constants .tau..sub.1 and .tau..sub.2 of MMV's 103 and 104 which are determined by fundamental and timing factors, and the time constants depend upon the temperature and the supply voltage. Accordingly, there is a problem that the revolution speed of the rotating body is deviated from the predetermined value due to variations in ambient temperature and in supply voltage.
Further, in the conventional frequency discrimination circuit shown in FIG. 2, the pulse signal A generates the error voltage E equal to the voltage E.sub.0 at a plurality of periods on the basis of the characteristic of the circuit. Accordingly, when a servo system for controlling the speed of revolution of the rotating body is constructed using the conventional frequency discrimination circuit, there is a danger of the rotating body being locked in a stationary state at a plurality of speeds of revolution. In more detail, in the case where the revolution speed of the rotating body 2 is greatly increased for some causes and the period T of the pulse signal A is smaller than the time constant .tau..sub.1 of MMV 103, as shown in FIG. 4, a so-called frequency dividing effect is generated by MMV 103, and input pulses applied to MMV 103 before the lapse of a delay time corresponding to the time constant .tau..sub.1, that is, input pulses a shown in FIG. 4 do not trigger MMV 103. As a result of inhibiting the input pulses a, the repetition period of the trapezoidal wave signal D becomes much larger than tee period T of the pulse signal A. On the other hand, the pulse signal P for sampling the signal D has the same period as the pulse signal A, and there is little probability that the leading part (that is, the constant inclination part) of the trapezoidal wave signal D is sampled. Accordingly, a maximum value and a minimum value (equal to zero) of the signal D are alternately sampled to produce the error voltage E. A mean value of the error voltage thus sampled is equal the about one-half the maximum value of the signal D, that is, nearly equal to the voltage E.sub.0. In other words, the servo control system is brought to a lock-in state also in this case. The pulse signal A produces the error voltage E equal to the voltage E.sub.0 at different periods, and thus there is a danger that the revolution speed of the rotating body 2 is pulled in different values, that is, into false stable state of the control system.
It is to be noted that, since the driving motor 1 employed in the servo control system has an integration effect, the motor 1 is operated as if driven substantially by a mean value of the waveform E (error voltage E) shown in FIG. 4.